Monitoring Tensile Load in a Bolt

The present invention relates to methods and apparatus for monitoring the tensile load of nut and bolt assemblies as an indicator of the tightness of the nut on the bolt, and in particular to detecting unintentional slackening of a nut on a bolt.

The torque with which a nut is tightened onto a bolt is normally critical in ensuring the long-term security of the connection formed thereby. Unintentional slackening of a nut can occur in many applications, due, for example, to vibrations experienced from the bolt from the environment in which it is used. Such slackening of a nut can allow parts which are connected together by the nut and bolt to move relative to each other, and, ultimately, to completely separate from each other, which can, in turn, lead to catastrophic results, such as a wheel coming off of a vehicle causing it to crash, sections of train track separating from each other causing a train to crash or in civil engineering structures causing collapse, and the like.

Slackening of the clamping force of a nut can occur for many reasons. For example, it is known that the fish plate will move during expansion of the railway track. The clamping force can be quite high, but track movement induces wear which has the effect of loosening the clamping force. This force is applied by a threaded bolt and nut. Any change in the distance between the nut and the bolt head will cause a significant loss of clamping force, even though this figure will be very small.

One approach to overcoming this problem has been to develop locking nut systems which are designed to make it more difficult for the nut to unintentionally slacken, such as by provision of secondary nuts of opposite thread which interact with a primary nut to prevent unintentional rotation.

Another approach has been to develop electronic systems which measure the tightness of the nut and wirelessly transmit the information to a remote monitor which issues an alert when a nut is detected to have slackened more than an allowable amount. In particular, one such system known in the art utilises a strain sensing washer which is compressed by the nut as it is tightened, the strain in the washer when the nut is fully tightened being recorded as the datum setting. If the nut slackens off, the strain in the washer will reduce, and if the strain in the washer falls below a pre-set value, an alert to check the nut issued. Although this system generally works acceptably, due to the strain gauges which must be provided on every washer fitted on each joint, the cost is relatively high.

There is, therefore, a need for a system for monitoring the load in a nut and bolt assembly which is a low cost so as to make it cost effective to implement in large scale applications.

One object of the present invention is to provide a method and apparatus for confirming that there has been no change in the clamping force of a correctly assembled nut and bolt assembly. It is a further object of the present invention to provide a system which can be implemented in a cost-effective manner.

One solution of the invention relies on the measurement of the resonant frequency of the assembly i.e. nut, bolt, fish-plate etc. To measure the resonant frequency which will be in the inline axis of the bolted assembly, a non-contact distance over time measurement system is envisaged. This may take the form of a laser which is directed at the end of the bolt with a high resolution, possibly microns, eddy current detection or other contactless distance measuring techniques. In order to excite the bolt, an external force is required. This is preferably achieved by a magnetic field, for example produced by a solenoid which is arranged to be coaxial to the bolt and nut. Application of current in the solenoid coil at the correct frequency will cause a resonance in the bolted assembly. This frequency is best provided by a multiplicity plat of frequencies generated by noise. This noise should have components which are equal to the resonant frequency, i.e. at least twice the expected resonant frequency.

According to a first aspect of the present invention there is provided a method of measuring the resonance of an article comprising the steps of directing a laser onto the article, detecting the reflected laser signal from the article, using the reflected signal to calculate the distance between the laser and the article, monitoring changes in the calculated distance over time due to vibration of the article, and using the change in distance between the laser and the article to calculate the frequency of vibration of the article.

The present invention further provides a method of measuring the resonant frequency of an article comprising the steps of exciting the article to vibrate at its resonant frequency, measuring changes in the distance over time between the article and a reference point due to the vibration of the article, and calculating the vibrational frequency of the article based on the changes in the distance between the article and the reference point over time.

Preferably, the article is excited by placing it in a varying magnetic field, such as, for example, by using an exciter coil. This may excite the article simply by the magnetic effect inherent in the article. However, in order to enhance the coupling between the solenoid and the bolt, a magnet could be inserted into the bolt which would improve the coupling of the magnetic field, generated by a solenoid and the magnet, which could be of the rare-earth type, i.e. samarium cobalt, neodymium iron. Many different forms or types of low cost magnets are available apart from the above. These could take the form of circular or square section. This has the advantage that the only modification which may be required to the article is the addition of a magnetic, which is a very low cost item, and the remainder of the electronics required for implementation of the method can be included in a detector which can be used with multiple articles.

The distance measurement may be effected by means of any known contactless distance over time measurement system. For example, a laser may be used to measure the distance between the article and the laser, a processor recording changes in the distance and calculating the corresponding frequency. Other techniques may also be used, including but not limited to eddy current technique for measuring distance.

An electronic identification means such as an RF ID tag may also be embedded into the article that each article can be uniquely identified so allow easy remote identification of an article which requires attention due to the loss of bolt preload or the like. The present invention further provides a detector for a bolt tensile load monitoring system comprising a magnetic field generator for exciting a bolt to be monitored, and a contactless distance measuring arrangement configured to measure variations in distance over time between the detector and the bolt due to the vibration of the bolt.

The contactless distance measuring arrangement may comprise a laser configured to measure distance over time in a known manner, an eddy current based distance measuring device or other known devices for contactless distance measurement. The only restriction is that the response rate of the arrangement must be sufficiently high to be able to detect the changes of distance over time associated with the range of frequencies at which the device will resonate.

Preferably, the magnetic field generator comprises an exciter coil which may be located around the bolt or which may extend a magnetic field to envelope the nut and bolt assembly. The magnetic field generator may include decoupling means such as a decoupling sleeve to ensure that the mass of the magnetic field generator does not change the resonant frequency of the article which is being monitored. The detector may also include a reader for reading an electronic identification means associated with the bolt, such as an RF TAG antenna, and may also include a temperature sensor for detecting the temperature of the bolt and allowing temperature compensation of the frequency measurements to be made.

The system of the invention operates by detecting changes in the resonant frequency from a datum value, so that a reading is taken when monitored characteristic of the article, such as tensile load of a nut and bolt arrangement, is at the correct value, with variations in the measured resonant frequency indicating a change in that monitored characteristic.

The invention may be implemented as a hand held, portable detector or may be realised as a larger scale detector, which may, for example, be mounted or retrofitted on a maintenance vehicle such as suitably equipped rolling stock, aerial drones or static remote sensors to detect by pinging and, if necessary, wake up the device to permit the instantaneous reading of the data being measured and monitored.

According to a further innovative development in the field of the above detailed invention, there is provided a cap for a nut which, in use, is releasably mountable on a nut, the cap including sensor means for sensing the tensile load on the nut and bolt assembly as a measure of the tightness of the nut on a bolt, and means for transmitting the tensile load sensed on the nut and bolt assembly to a remote location for monitoring changes in the tensile load.

A cap in accordance with this invention has the advantage that it btrt incorporates load monitoring means into a cap which is retained on and normally left on the cap, together with transmitting means for communicating the tensile load information to a remote location, the tightness of a nut can be continuously monitored in a cost effective manner from a remote monitoring station without requiring regular local checks, either my means of manual checks of the torque in the nut or by use of a local reader to check the tensile load using an electronic system of the type outlined above and detailed hereinafter. This enables slacking of a nut to be identified promptly and in a cost-effective manner. Furthermore, the cap may be designed to fit on any conventional nut and bolt arrangement, allowing retro-fitting to existing installations.

The cap may advantageously include power up and power down algorithms to conserve energy/ battery power. For example, when the article senses bolt preload within prescribed limits, the system is powered down into a low power mode or when a detector is within range, the system is powered back up to activate the cap and its interrogation systems.

The present invention offers numerous advantages over the prior art. Manual inspection frequency, such as by walking a track, is reduced, possibly being required only when an alert signal from the system of the invention is received. The system also offers a wide range of options for alerts to be communicated, such as by a computer, a hand-held device / phone etc. The constant monitoring options offered by the system also facilitates more accurate lifetime prediction and replacement cycles, as well as confirmation that a nut has been correctly installed and correctly tightened / torqued.

The system also helps identify problem areas where loosening might be more of a problem and also help extend the working life of associated assets such as joints, switches and the like by preventing excessive wear due to movement resulting from loosening of nut and bolt arrangements. The nut and bolts of the invention can also be uniquely identified and located for easy maintenance, and can form part of an accountability system for tracking proper maintenance and inspection has been carried out. The data gathered could be useful in accident investigation.

The cap of this invention may be configured to communicate with a load sensing washer against which the nut is tightened, the cap including a circuit board or the like which communicates via a waterproof wired electrical connection with the load sensing washer to read the load on the washer as a measure of the tightness of the nut on a bolt and then broadcast the data to a remote monitoring location.

Alternatively, the cap may include exciting means which operates to excite the nut and bolt on which, in use, the cap is fitted at its resonant frequency, as well as a reader which measures the resonant frequency of the nut and broadcasts the data to the remote monitoring station, which monitors for changes in the measured resonant frequency as an indication of a change in the tensile load on the nut. This may be caused by a slackening or a tightening of the nut, either of which may require investigating as excessive tightening can cause failure of the joint formed by the nut due to shearing or the like and hence be of as much concern as unintentional slackening.

In particular, the cap may include an exciter coil which develops a magnetic field around the nut and bolt assembly to cause it to resonate. As described above, this may rely on the natural magnetism of the material from which the nut and/or bolt on which it is screwed is made or a magnet may be embedded into the nut or bolt.

The resonant frequency may be measured using one of the techniques described above in connection with the invention. In particular, a laser may be embedded in the cap which measures variations in the distance of the nut or another part of a nut and bolt arrangement from the laser which is then used to measure changes in the resonant frequency as described above.

Preferably, a PCB is mounted onto the cap, in particular attached to the axial end thereof, on which is mounted a surface mounted laser or other distance measuring means. Wireless transmitting means such as a WIFI or Bluetooth module may also be incorporated on the PCB or provided on a separate circuit board. An antenna to extend the range of the wireless transmitting means may also be incorporated into the cap, either extending longitudinally there along or around the inner cylindrical surface thereof. A power source such as a button cell, piezo-electric or inductive energy harvesting device may also be mounted in the cap underneath the PCB. Preferably, an exciter coil is then mounted on the inner cylindrical surface of the cap so as to extend around a nut on which the cap is fitted.

The cap preferably includes suitable means for releasably retaining it on a nut and/or bolt. In one embodiment, it may include one of an internal spring clip and a circumferential groove, a mating nut having the complementing other of the spring clip and groove such that when the cap is fitted onto the nut, the spring clip engaged in the groove to retain the two parts together. Alternatively, the cap may have a hole in the end through which the end of a bolt may protrude and have a retainer attached to prevent relative axial movement between the cap and the bolt.

The cap may particularly advantageously also operate as a locking cap, of a type known in the art, which engages with a part of locking nuts fastened onto a bolt so as to prevent relative rotation therebetween.

Another invention which forms a part of the disclosure of this application comprises a smart bolt arrangement, comprising a bolt having at least one nut screwable thereon, load sensing means associated with the nut and/or bolt for detecting at least changes in the tensile load on the nut, a cap releasably securable over the nut so as to be normally retained in position on the nut, electronic means associated with the cap for measuring at least the changes in the tensile load on the nut detected by the load sensing means, and wireless transmitting means associated with the caps for transmitting the measurements from the cap to a remote location.

Preferably the wireless transmitting means and the electronic means are housed within the cap. The cap preferably takes the form of the cap as described above and hereinafter and may include the same features described in relation thereto.

In order that the invention may be well understood, there will now be described an embodiment thereof, given by way of example, reference being made to the accompanying drawings, in which: Figure 1 is an illustrative view of a bolt and probe suitable for implementation of the method of the invention as a bolt tension measurement system;

Figure 2 is a block diagram of a system suitable for implementing the bolt tension measuring system of Figure 1 ;

Figure 3 is a side exploded view of a smart bolt by means of which the bolt tension measurement system utilising an excitation magnet and energising coil may be implemented;

Figure 4 is a side assembled view of the smart bolt of Figure 3;

Figure 5 is a side exploded view of a smart bolt by means of which the bolt tension measurement system utilises a load sensitive washer and a wired connection to processing electronics; and

Figure 6 is a side assembled view of the smart bolt of Figure 5.

Referring to Figure 1 , there is shown a nut 2 and bolt 1 of one, but not the only type with which the invention may be used. It will in particular be understood that a conventional nut and bolt assembly would similarly benefit from the invention. The particular nut and bolt arrangement shown in a locking nut system of the type disclosed in applicant's own earlier British Patent No. GB2367600, in which the bolt 1 is provided with a dual thread, with a first, clockwise section of the thread having a primary nut 2 screwed thereon, and a second, counter-clockwise section of the thread having a secondary nut 3 screwed thereon which acts as a lock nut for the primary nut 2. A locking cover 4 is then mounted over both nuts to lock them against rotation relative to each other and hence relative to the bolt 1 . It will, however, be understood that the invention is not limited to application with this nut and bolt arrangement and may be used to monitor the tension in any nut and bolt arrangement, and, indeed, the principles of the invention may be used to monitor changes in the natural frequency in any article to identify a change in its structural stiffness / integrity.

The bolt 1 has a magnet 5 embedded in it in order to enhance its responsiveness to a magnetic field, although the invention may be implemented based only on the natural magnetism of the material of the bolt. An RF ID Tag 6 is also embedded in the bolt 1 which may be read by a remote reader so as to enable the bolt to be identified and datum reading to be stored and recalled.

Also shown in Figure 1 is a hand held bolt probe 10 which is an embodiment example of one type of instrument which may be used to implement the invention. The probe 10 has a hollow former 1 1 with an opening 12 in one end which is sized to enable the end of the bolt 1 , with the nuts 2, 3 and cover 4 secured thereon, to be received inside the former so that the end of the bolt is located in a magnetic field induced by a exciter coil 13 wound around the outside of the former 1 1 .

The body of the probe 10 has a laser 14, as an embodiment example of a suitable contactless distance measuring device, mounted in it which points into the opening 12 in the former 1 1 , the laser being configured to measure the distance from the laser to an article such as a nut located inside of or in front of the former 1 1 in a manner well known in the art. A handle 15 is provided on the probe to allow easy positioning of the probe onto the end of the bolt 1 .

In order to measure the load on the bolt, the primary nut 2 is first tightened to the correct torque and the locking nut 3 and cover 4 fitted. The probe 10 is then fitted into the end of the bolt 1 with the cover 4 of the nut arrangement located in the opening 12 in the former 1 1 and the laser directed at the end of the bolt 1 which is visible through a central aperture 4a in the cover 4. The probe is then activated by passing a current through the coil 13 which induces a magnetic field inside the former which causes the nut and bolt arrangement to resonate. This resonance of the nut and bolt causes minute variations in the distance between the end of the bolt 1 and the laser which are measured by the laser and converted by a computer into a resonant frequency reading which is recorded for the particular nut a bolt arrangement.

In the illustrated embodiment, the probe 10 also has an RF ID Tag reader 17 which reads the unique serial number for the bolt 1 which is stored in the RF ID tag 6 so that the readings for the bolt can be stored against the serial number of the bolt. The datum or calibration reading may also be stored in the RF ID Tag. In order to check for slackening of the nut on the bolt, the probe is simply re-fitted over the cover 4 and the coil 13 re-energised so as to excite the nut and bolt arrangement, and the frequency of the induced vibration again measured using the laser. Since the laser is simply measuring variations in the distance it is not necessary that the probe be re-positioned exactly as it was for the datum reading or even for the same probe or probe arrange to be used. The measured frequency is then compared with the datum reading to see if there has been any change. Slacking of the nuts 2, 3 will cause a change in the overall tension of the assembly which will, in turn, changes the natural frequency of the assembly, so that any change in the frequency of vibration measured by the probe 10 can be used to calculate the degree of slackening of the nuts.

Changes in temperature can, of course, also cause changes in the vibrational frequency due to thermal expansion and contraction, and in order to compensate for this, the probe 10 also includes a temperature sensor 16, such as an IR sensor, which can be used to provide temperature compensation to the frequency readings taken by the probe.

The probe may also include communication means such as GSM, RF, satellite or other means by which data can be communicated to and from a remote monitoring station.

Although the invention has been described in connection with a handheld reader and for measuring the tension in a nut and bolt arrangement, it will be understood that the probe may be implemented in other forms depending on the applications with which it is to be used - for example it would be impractical to use a handheld probe to check the tension in retaining nut and bolt assemblies on multiple joints with numerous bolted connections due to the number involved.

It will further be understood that the invention does not have to be implemented with a laser as the distance measuring arrangement, and other systems, which may be optically based, acoustically based or based on other known systems such as eddy current techniques, may also be used.

It will also be understood that while the invention has been described in connection with monitoring changes in tensile load in joints held together by nuts and bolts, the principle of the invention of using a laser to measure frequency of vibration may be used for any application or structure where monitoring the change in its resonant frequency from a datum / calibration value can be used to indicate a change in the stiffness / integrity of the application / structure.

The invention may also be implemented as a permanent addition to a structure for real time monitoring of the characteristics of interest.

Referring now to Figures 3 and 4, there is shown an embodiment example of a smart nut and bolt assembly 20 which illustrate further innovations which may be used to implement the bolt tension measurement system of the above described invention.

The smart bolt and bolt assembly 20 comprises a conventional bolt 21 on which is screwed a convention nut 23 in order, in use, to secure a part (not shown) in place between the nut 23 and the bolt 21 . In the illustrated embodiment, the nut and bolt arrangement is applicants own patented locking nut system which utilises two nuts 23, 24 of opposite threads which screw onto oppositely threaded parts 22a, 22b of the shaft 21 and are then locked together by a cap 26 in order to prevent relative movement between them and thereby prevent unintentional slackening of the nuts 23, 24. It will, however, be understood that the smart bolt may be implemented with just a standard conventional nut and bolt.

In the smart nut and bolt assembly 20 of the invention, the cap 26 includes a sensor 27 which monitors and wirelessly transmits changes in the tensile load on the nut and bolt assembles 23, 24. In the illustrated embodiment, the system monitors the resonant frequency of the nut and bolt assembly 20 and uses changes in the resonant frequency to indicate a change in the tensile load in the nut and bolt assembly in a manner described above in connection with Figures 1 and 2.

To achieve this, a coil 28 is mounted on the inner circumferential surface of the cap 26 which generates a field around the end of the bolt 21 which are engaged within a hermetically sealed plastic electronic component carrier, securely mountable within the cap. The generated field by means of the electrically energised coil 28 agitates the magnet 25 which in turn is attached to the nut and bolt assembly 20 causing the whole assembly to resonate, which is detected in the cap sensor 27 by measuring variations in the distance of the end of the bolt from a reference point in the cap 26. As described above, the resonant frequency will change depending on the tightness of solely the nut 23 or a combination of nuts 23 and 24 when fitted.

In order to measure the distance of the end of the bolt from the reference point, and therefore the vibration of the bolted assembly, in the illustrated embodiment, the sensor 27 includes a surface mounted laser mounted on a circuit board 27a which is mounted inside the cap 26 against the closed end thereof. The circuit board 27a also includes a temperature sensor so as to allow temperature compensation of the readings to be carried out in a well-known manner, an RFID tag and are connected to a power source in the form of a button cell or piezo-electric or inductive energy harvesting device 27b which is also mounted in the cap 26 as part of the sensor 27. Power source 27b also provides power to the coil 28. Finally, the sensor includes a WIFI module 27c which enables wireless communication to a remote monitoring station 31 for transmitting data from the sensor 27 for remote monitoring. Although the sensor is described as being mounted inside the end of the cap 27, it will be understood that it may instead by mounted on the outside of the end of the cap 27.

In order to retain the cap 26 in place on the nut 23, and 24 (where fitted), a circumferential groove (not shown) is formed on the inner circumferential surface of the cap 26 which engages with a deformable ring member such as a C-clip which is mounted on the outer surface of the nut 24, the C-clip resiliently deforming inwards to allow the cap 26 to be engaged onto the nuts 23, 24 and then spring back out to engage in the groove in the cap 26 to impart a restraining force which prevents the cap being removed until a force of a certain level is applied.

Referring next to Figures 5 and 6, there is shown an alternative embodiment example of a smart nut and bolt assembly of the invention. In this case, in place of the resonance based monitoring system, a load sensitive washer 29 is instead provided between the nut and the bolt which monitors the preload on the nut and bolt. The load washer 29 is then hard wires to the electronics package housed within the cap 26 which processes the signals from the washer 29 and transmits the information to a remote reader in the same manner as described above.

It will be understood that the smart nut and bolt assembly described above enables real time monitoring of a number of joins at the same time. Each sensor will include a unique electronic identifier such as a unique RFID tag as 27a a part of electronics package 27, so that the data for each nut can be identified and slackening of a particular nut easily and quickly communicated from the monitoring station to the appropriate personal for prompt action.

Although the smart assembly has been described in relation to the laser based resonant frequency monitoring system it will be understood that the inventive aspects of the smart assembly is not that particular system and it may be used with other resonant based monitoring systems. Furthermore, it may also be used with other wireless load monitoring systems. For example, the sensor 27 may communicate with a load measuring washer, of the type known in the art, which is clamped between the nut and bolt and its output signal is fed by a wired connection 30 to sensor 27.